The present invention relates to an ink jet-recording ink as well as a method for making a lithographic printing plate which makes use of the ink jet-recording ink. More specifically, the present invention pertains to an ink for the ink jet-recording, which is highly susceptible to the irradiation with a radioactive ray, capable of forming an image of high quality and excellent in the storage stability and a method for preparing a lithographic printing plate, which never requires any developing treatment and which can form an image excellent in the printing durability and having high image quality.
There have been known, for instance, the electro-photography technique, the sublimation or melting type heat transfer printing technique and the ink jet-recording technique, as the image-recording techniques in which an image is formed on a substrate such as a sheet of paper on the basis of image data signals. Among them, the ink jet-recording technique comprises the step of injecting an ink only on the desired image areas of a substrate to thus directly form a desired image on the substrate and therefore, the technique permits the efficient use of such an ink. This technique is also excellent in that the structure thereof is very simple and that it only generates reduced noise.
There have been adopted, for instance, a method for drying an ink in the air to thus remove the solvent from the ink and a method which makes use of an ink curable through the irradiation with a radioactive ray such as ultraviolet light rays for the immobilization of the injected ink on the surface of a substrate, in the image-recording method according to such an ink jet-recording system. In particular, the ink jet-recording ink curable through the irradiation with a radioactive ray such as ultraviolet light rays (radiation-curable ink jet-recording ink) only gives out a slight smell and is excellent in quick-drying characteristics and thus permits the formation of an image even on a substrate which has a poor ink-absorbing ability, whose direct recording is in general quite difficult, without being accompanied by any bleeding, while holding excellent sensitivity and which permits the formation of an image excellent in the adhesion to the substrate. For instance, in the patent article 1 as will be specified later, there is provided an ink jet-recording ink having the following desired properties in addition to the foregoing: it has only a reduced stimulation and sensitization even when it is applied onto the skin and it is thus excellent in safety. When using such a radiation-curable ink jet-recording ink, the ink should satisfy the following requirements such that it must have a sufficiently high sensitivity to the a radioactive ray used and it can form a high quality image after curing the same. Such an ink would have a variety of advantages, for instance, it can ensure a sufficiently high curing ability, it can reduce the electric power consumption required for the source of a radioactive ray used for curing the same, it can extend the service life of the source of a radioactive ray through the reduction of the load applied thereto and it can prevent the generation of any low molecular weight substance due to the insufficient curing of the ink, insofar as the ink has a sufficiently high sensitivity to a radioactive ray. In particular, when this ink jet-recording ink is used for forming image areas of a lithographic printing plate for use in the offset printing, the improvement of the sensitivity of the ink would permit the considerable enhancement of the strength of an image after the curing thereof and the resulting lithographic printing plate would thus have high printing durability.
In general, methods, which make use of a variety of polymerization initiator systems, have been proposed as means for improving the sensitivity, to a radioactive ray, of a radiation-curable, polymerizable compound (see non-patent article 1 specified later). Generally used include, in particular, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, and 2-ethyl anthraquinone (see non-patent article 1). However, the use of these polymerization initiator systems would reduce the susceptibility, to curing, of the polymerizable compositions containing the systems such as ink jet-recording inks and this accordingly requires the use of a longer exposure time for forming an image. For this reason, in case of a fine image, any image of good quality cannot be reproduced when there is a slight vibration during image-forming operations and the radiant energy from the source thereof should considerably be increased and accordingly, one should devise any means for radiating or eliminating a large amount of heat proportional to the increase in the radiant energy. In addition, a problem also arises such that the film of the composition is liable to undergo deformation or any change in quality.
In addition, there has also been known such a method which comprises the use of a combination of a sensitizing dye and a polymerization initiator as the foregoing polymerization initiator system to thus improve the sensitivity of the foregoing composition through the interaction between these components. However, the use of such a polymerization initiator system would be attended by such a drawback that the image obtained by polymerizing the ink jet-recording ink containing the system has a reduced storage stability.                Patent Article 1: Japanese Un-Examined Patent Publication 2003-192943        Non-Patent Article 1: Bruce M. Monroe et al., Chemical Revue, 1993, 93: 435-448.        